1. Field of the Invention
The present invention is directed to a process for chiral cyanohydrination in which catalyst poisons or impurities are removed to increase enantiomeric selectivity.
2. Description of the Prior Art
Chiral cyanohydrins, including optically-active alpha-hydroxy benzonitriles are known in the art and are of interest, per se, and as intermediates, e.g. to esters. In pyrethroid esters, those having an alpha-S-alpha-hydroxynitrile moiety coupled with the appropriate pyrethroid acid usually have the highest pesticidal activity. However, such alpha-S-alpha-hydroxynitriles have not been particularly easy to obtain in the past because they were usually prepared by resolution.
Asymmetric synthesis of R-mandelonitrile by addition of hydrogen cyanide to benzaldehyde in the presence of a synthetic dipeptide catalyst is known in the art, as in Oku, Jun-ichi and Shohei Inoue, J.C.S. Chem. Comm., pages 229-230 (1981), and other Oku publictions where, e.g., cyclo(L-phenylalanyl-L-histidine) containing 1/2 mole of water of crystallization was used. However, it has been found by applicants that the process of chiral cyanohydrination with cyclo(L-phenylalanyl-L-histidine) or cyclo(D-phenylalanyl-D-histidine) is not necessarily satisfactory or optimal for the preparation of chiral cyanohydrins because of problems with the purity of the aldehyde used for the process associated with poisoning of the catalyst and competitive non-chiral reactions. Applicants have now found that these problems can be solved by the present invention while also increasing further the enantiomeric selectivity of the chiral cyanohydrin product.